1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs post-processing on sheets formed with images, and relates to an image forming system having the sheet processing apparatus.
2. Description of the Related Art
Heretofore there have been widely known sheet processing apparatuses that perform post-processing such as folding on sheets output from an image forming apparatus such as a copying machine or a printer. Sheet processing apparatuses have become more multifunctional in recent years. Some of them have an end stitching function to stitch a sheet bundle at its end, a saddle stitching function to stitch a sheet bundle at its center, and a bookbinding function to perform bookbinding by folding a saddle-stitched sheet bundle at its saddle-stitched part.
In Japanese Laid-open Patent Publication No. 62-16987, a sheet folding apparatus has been proposed in which a bundle of sheets fed from a copying machine is inserted into between a roller pair and thereby folded into two, and only a folded part of the sheet bundle is pressed by moving an additionally folding roller (press roller), with the folded part held between the roller pair, whereby the sharpness of the folded part is ensured.
When enhanced folding is performed on a sheet bundle as described above, stacking of sheets on a sheet stacking tray, folding of the sheet bundle conveyed from the tray, and pressing of the folded part of the sheet bundle are sequentially performed. To improve the productivity, the folding of the sheet bundle is sometimes performed concurrently with the staking of the next sheet bundle on the sheet stacking tray.
However, if the number of sheets that constitute a sheet bundle following a sheet bundle to be processed is small and a time required to stack the next sheet bundle is short, a time required to perform the press operation on the sheet bundle to be processed is longer than the time required to stack the next sheet bundle. In that case, an operation for folding the next sheet bundle cannot be started until the press operation of the sheet bundle to be processed is completed. It is therefore necessary for the next sheet bundle to wait on the sheet stacking tray, and the productivity is lowered accordingly.